Wood and biomass have been converted to paper and packaging carton products by chemical pulping methods since 1864. The aim of chemical pulping is to delignify biomass and release fibrous cellulose material. Cellulose is composed of straight glucose chains consisting of several thousand units. These fibers are extremely strong, but only 1-3 millimeters long and only nanometers wide. When fibers are formed on a flat surface and dewatered, the intertwined fibers create a mat of pulp or paper sheet. The strength of a sheet from pure cellulosic fibers is weak due to lack of chemical bonding.
The hemicelluloses are short chains of only few hundred monomer units. The hemicelluloses consist of heterogeneous chains of sugars including glucose, xylose, mannose, arabinose, galactose, and rhamnose. In addition, the some hemicellulose side chains terminate in acetyl groups, which gives them functional properties. The hemicelluloses bond between the cellulose fibers creating a strong network, a fundamental property of paper. Other hemicellulosic components, such as uronic acids, are not desirable because they cause chemical consumption in pulping and bleaching.
Commercial paper machines form paper in a continuous forming table, named after Fourdrinier. Because the table is moving at ever-increasing speeds, the fibers orient in the machine direction giving more strength than the cross-machine direction. Higher hemicellulose content increases the strength in both directions. In some cases, the bonding is increased by adding starch in the Fourdrinier or on the dried paper sheet. Starch is a sugar polymer of vegetable origin.
Pulping processes are broadly divided into mechanical, semichemical, and chemical methods differing in pulp yield. Chemical pulping dissolves about half of the original material to release bleachable cellulose. This dissolved material is commonly burned to recover chemical used in the pulping. Alkaline pulping chemicals include sodium hydroxide, sodium carbonate, and sodium sulfide in different variations. Acid-based pulping chemicals include sulfur dioxide as sodium sulfite, ammonium sulfite, calcium bisulfite, and magnesium bisulfite in acid-sulfite pulping.
Mechanical pulping utilizes grinding wood or woodchips by mechanical means. An example is the stone groundwood process, where a complete wood log is ground to fine powder with nearly 100% yield. This requires large amount of electricity, such as 1500 to 2500 kWh per ton of wood. Thermomechanical pulping systems incorporate a short presteaming step before disk refining of wood chips.
Semichemical pulping is a compromise between chemical and mechanical pulping. The yield is typically 75-85% from a 10-60 minute cook. The aim is to soften the wood chips enough to significantly reduce refining energy, while maintaining the hemicelluloses in the pulp. A portion of lignin is dissolved in the sodium-based or ammonium-based processes. The process may include acidic, neutral, or alkaline pulping conditions. Sulfur dioxide in the form of ammonium sulfite or sodium sulfite may be used in the neutral and acidic processes. Sodium hydroxide or sodium carbonate may be used in alkaline processes.
In semichemical pulping, wood chips are first subjected to mild cooking in, most commonly, sodium sulfite combined with a small quantity of alkaline salts, such as sodium carbonate, sodium bicarbonate, or sodium hydroxide. The cooked chips are then sandwiched in a disk refiner—or two rotating serrated disks—that separate the individual fibers of cellulose. The pulp is then washed to remove the chemicals.
Semichemical pulping is commonly performed in a continuous digester in the absence of free liquid. This is called “vapor phase” digesting. Washing is performed in 2-5 stages to remove 70-95% of the dissolved solids from the pulp. This is important for economic and environmental reasons. The digester and washers present approximately half of the pulp plant cost.
The pulp from semichemical pulping is used for packaging products, especially for corrugating medium in box plants. Semichemical pulping results in stiff fibers, and the process is used to make corrugated paperboard, cardboard roll cores, and containers. Important physical properties include crush resistance of the corrugators. Preferred fiber sources are hardwood and bagasse, which contain shorter fibers. The hemicelluloses are retained to add bulk and strength from bonding. Lignin removal is not necessary for unbleached products. Higher yields are preferred to reduce cost of fiber.
The semichemical pulping spent liquor contains reacted cooking chemicals and dissolved wood components. The combustion of this liquor is practiced to recover chemicals and to produce energy for the process. Due to high pulping yield, the amount of energy recovered is less than energy needed for processing the liquor. In some cases, no process energy is produced, or in extreme cases, chemicals are not recovered. The sodium is typically recycled by combusting in a recovery boiler. The sulfur dioxide from ammonium sulfite can be recycled from flue gases after combustion of the spent liquor. In either case, the recovery process present about half of the pulping plant cost.
What are desired are improvements to semichemical pulping processes to reduce washing costs, recovery process costs, and overall process costs, while producing equivalent pulp and paper products.